Hard surface composite article and method of making



United States Patent Earl M. Anger, Scottdale, Pa., assignor to TheDuraloy Company, Scottdale,

Pa., a corporation of Delaware No Drawing. Application April 4, 1950,Serial No. 153,999

24 Claims. (Cl. 29-195) The present invention relates generally to theart of coating metal articles and is particularly concerned with novelmetal articles having surface portions of exceptional hardness anddurability, and with novel methods by which said articles can beproduced.

Commercial rolls, dies, guides and similar devices, which are employedat elevated temperatures in steel handling and shaping operationsrapidly pick up metallic particles which firmly attach themselves tosaid devices. For example, the rolls on the shafts in roller bottomheating furnaces pick up metal from the sheets which they convey throughthe furnace and such picked up metal takes the form of minute sharppointed cones which adhere tenaciously to the rolls and form pits ordepressions in the sheets subsequently moving over the rolls. It isprohibitively expensive to free the rolls of picked up metal and therolls are too expensive to be replaced whenever they have picked up suchmetal. There has consequently been a long-standing urgent demand for asolution to this problem, which has not to my knowledge hitherto beenmet.

There has also been a long standing, urgent and heretofore, unsatisfieddemand for articles which would resist over protracted periods thecorrosive effects of molten aluminum, such as melting pots, skimmers andthermowells employed in the handling and treatment of molten aluminum.Ordinarily, iron, steel and steel alloy articles of this type arerapidly attacked and destroyed in ordinary commercial use. Likewise, ingalvanizing operations there has been a serious and heretofore unsolvedproblem of rapid corrosion and destruction of such metal equipment bymolten zinc.

In accordance with the present invention, all these problems can besolved. The articles of my present invention, for example, not only donot pick up metal particles in high temperature metal workingconditions, but they are so resistant to corrosion by molten aluminumand molten zinc that they can be used for periods many times longer thaniron, steel and steel alloy articles heretofore employed. In fact, incontact with molten zinc, rather than being eroded or corroded, thearticles of my invention are built up to a measurable but notundesirable degree, a blue powder being formed on them which tends toeven further increase their wear-resistance and normally long usefullives. Furthermore, the articles of my present invention have thesurprising characteristic of being useful in the production of plateglass, where they solve the old serious problems of pickup and sticking.My articles in the form of glass forming rolls do not either pickupparticles of raw hot glass stock or stick to it. Thus, this inventionpromises to save vast amounts of plate glass production expenses, thefrequent replacement and cleaning of glass processing rolls heretoforealways being essential to the production of satisfactory glass plates.

In addition to the foregoing advantages, the articles of this inventionarerelatively inexpensive to produce, compared to the devices of theprior art, and their manufacice ture in accordance with the method ofthis invention is relatively simple and free from critical circumstanceswhich are difficult to establish and maintain.

Briefly stated, a typical article of this invention comprises a suitablemetallic base portion and an aluminum alloy portion bonded tenaciouslythereto and comprising a plurality of layers of aluminum alloys ofdifferent compositions, the alloy portion being between about 0.005 inchand 0.040 inch thick and having an oxide-rich surface approachingdiamond hardness and having a diffinity for metal particles, saidsurface containing alumina as a preponderant oxide constituent and alsocontaining an oxide of metal of the base portion in a matrix of aluminumalloy.

The metallic base portion of the article of this invention may be of anydesired form as, for example, a roll or skimmer, a melting pot, athermo-well, or the like as mentioned above. Furthermore, this baseportion may be covered entirely or only partially with the aluminumalloy portion, depending upon the: portion of its area that is to besubjected in use to circumstances conducive to corrosion or undesirableaccretion or sticking, as above stated.

The base may suitably be of any of a number and variety of metals andalloys. The first and foremost criterion of a suitable metal or alloyfor the base is that this metal or alloy be of melting point temperaturesubstantially above the melting point temperature of pure aluminum andpreferably that it be above about 1400" P. which as will be explainedlater, is about the minimum heat treating temperature capable ofproducing the foregoing results regardless of the type or kind of metalor alloy present in the base. In the preferred practice of thisinvention, I employ a commercial alloy steel of chromium and nickel inwhich nickel content is approximately 35% and the chromium 15%, thebalance being primarily iron of the usual carbon content of about 0.2%with additional minor amounts of the usual impurities such as manganeseand silicon. It is, however, entirely possible to obtain the foregoingresults by using other alloy steels, such as chromium steel containingapproximately 12% chromium, and to use, for example, simple carbonsteels ranging in carbon content from about .09% to 1.5%. Commerciallypure iron, or even absolutely pure iron, is also suitable as aremalleable, white and gray cast irons providing that the grain size ofthe graphite in the latter is not so unusually large as to interferewith bonding of the aluminum alloy portion to the base article andalloying across the interface of the base and aluminum alloy coating. Inaddition to the ferrous metals, copper, bronzes, nickel and nickelalloys such as Inconel, Nichrome and the like, and chromium-nickelalloys can be used to gain the advantages of this inventron.

The base article can take the form of a massive solid body or a merecoating on a core of some other composition, the importantconsiderations being that the base have the ability to accept thealuminum coating and the ability to alloy with the deposited aluminum toproduce the tenacious adherence of the aluminum alloy layer required ingeneral commercial use of the final article.

In the preferred practice of this invention, the base article comprisesa core of ferrous metal and a coating of suitable alloy as describedabove. Accordingly, the first step in the method of this invention inthe preferred practice is the spraying of the ferrous metal. core withan oxidation-resistant alloy steel selected from the group consisting ofnickel steels, chromium steels and chrtr mium nickel steels and therebycoating the core.

Preliminary to producing the aluminum alloy portion on the base, thebase, regardless of its composition or form is suitably prepared toassure tenacious adherence of the aluminum alloy produced in accordancewith the subsequent steps of my process. This preparation, preferablyinvolves roughening the base surface and thoroughly cleaning it, as bysand-blasting, pickling, etching or some equivalent operation. In someinstances, however, satisfactory preparation of the base surface may beaccomplished by merely heating it to between about 200 F. and 500 F.,providing the aluminum coating is applied while the temperature of atleast the surface portion of the base is within this temperature range.

The production of the aluminum alloy portion or coating is accomplishedby metal spraying molten of heat-softened aluminum on the base articleprepared as described above. The metal spraying is done in accordancewith the usual commercial practice and is followed by a heat treatmentstep, which is essential to the production of an article having thequalities and advantages set forth above. These circumstances of thisstep are important, particularly so far as minimums are concerned, butthey need not be varied in accordance with such factors as filmthickness so long as the thickness of the aluminum film deposited on thebase article is within the range between about 0.005 inch and about0.040 inch. Films which are outside this range, however, are notdesirable as they cannot be relied upon to produce consistently theforegoing results. These heat treatment circumstances are preferablyvaried somewhat with variations in the constitution of the metal base,although generally speaking, satisfactory results can be obtained wherea deposited aluminum film of the requisite thickness is subjected to atemperature between about 1400 F. and 1800 F. for a period between aboutfive hours and about seven hours under an oxidizing atmosphere, that is,an atmosphere containing about 15% to 30% oxygen. If the base is ofcopper or a bronze, I prefer to limit the temperature of this treatmentto a maximum of about 1600 P. But where the base is of ferrous metal, i.e. iron, steel, steel alloy or cast iron, I prefer to subject thecoating to a temperature between about 1500 F. and about 1800 F. Also inthe better practice of this invention, the objects regardless of theconstitution of the base article are furnace-cooled gradually to roomtemperature under the same atmosphere as that to which they aresubjected in the previous heating period. It will be understood,however, that satisfactory results and articles of my invention may beobtained when the heat treating step is carried out at temperaturessubstantially above those stated above and/ or for periods substantiallylonger than five to seven hours, providing the base is not softened ormelted or distorted too much.

The circumstances of the heat treatment are designed to produceextensive alloying of the aluminum of the coating with the metal ormetals of the base bearing the coating. It is also designed to producesuflicient oxidation of the aluminum of the coating and the metal ormetals of the base alloyed with the aluminum and present in the surfaceof the aluminum deposit so that an oxide rich surface portion isproduced and is present on the finished article as stated above. Thus,since copper and tin alloy more readily with the aluminum than doesiron, chromium or nickel, the temperature range in this heat treatmentmay be and preferably is lower where the base article is essentially ofcopper or bronze than where it is of ferrous metal, nickel,nickelchromium alloy or chromium-nickel alloy.

I have obtained particularly good results where the heat treatment of analloy steel base having an aluminum coating in the above prescribedthickness range has been carried out in several temperature stages. Inthe first stage of this treatment, the coating is subjected to atemperature of about 1400 F. and maintained at that temperature forabout five hours. The coating is then subjected to a temperature ofabout 1600 R, which is maintained for about five more hours. Finally,the

temperature to which the coating is subjected is increased to about 2000F. and maintained for about five hours and then the object is cooled inair to room temperature. The article may be cooled between these variousheating steps, or any of them, but ordinarily no useful purpose isserved thereby and it is obviously not economical to do this. When theheat treatment is carried out as I have prescribed, there is produced inthe surface of the deposited aluminum coating or film a hard complexmixture of compounds. I am not certain as to exactly what takes place toproduce these compounds, but I know that in general, three well-definedlayers of aluminum alloys of different compositions are formed in thiscoating portion and that it is to these layers and especially theoutermost one that the extreme hardness, the resistance to pickup, andother advantages of my articles are attributable. In all the samples ofmy present articles that I have examined and analyzed, the first orinnermost layer which is bonded directly to the base article isprimarily a complex solid solution or alloy of aluminum and the metal orthe metals of the base, the aluminum being present in relatively smallproportions. The second or intermediate layer is materially richer inaluminum than the first, but also is an alloy of aluminum and the metalor metals of the base article. The outermost layer is rich in aluminumcompared with the others and is largely an alloy of aluminum with minoramounts of the metal or metals of the base article. I have previouslyhypothesized the existence of a very thin skin of aluminum overlyingthis outermost alloy layer but I have not been able definitely toestablish the existence of this skin. Instead I have found that all thealuminum in this outermost part of the coating is in the form ofaluminum-rich alloy, inter-metallic compounds with metal of the base andalumina, most of which is the alpha variety. I have further found thatthis outermost layer contains on its surface and immediately therebelowappreciable amounts of the oxide or oxides of the metal or metals of thebase. The alumina and the said oxide or oxides are disposed in a matrixof the aluminum alloy of the outer layer and they are tenaciouslyretained by this alloy. The total amount of oxide in the surface of thearticle of this invention normally amounts to between about 30% andabout 40% of the surface portion of the article. By surface portion Imean that part of the outermost alloy layer within about 0.0004 inch ofthe surface.

The hardness of the present articles is undefinable in exact absoluteterms because I have not been able to discover a polishing medium whichwill produce a sat isfactory surface for an accurate hardness test. Theworking surfaces of these articles are substantially harder than thesurface of similar articles heretofore known and used and they closelyapproach diamond hardness. Articles such as rolls prepared in accordancewith this invention, in fact, rapidly wear out emery wheels and the likewhich have been applied to them in attempts to effect polishing, andthese articles are sufficiently hard to withstand the most severewearing uses as rolls in roller-bottom heating furnaces, for example,without such erosion or deformation as will require their frequentreplacement.

The following illustrative, but not limiting, examples of the method ofthis invention are offered to assist those skilled in the art tounderstand and practice the present invention:

Example I An alloy steel roll for a roller-bottom heating furnacecontaining about 35% nickel, 15% chromium, 49% iron and 4% carbon wassand blasted and thereby thoroughly cleaned. Molten aluminum was sprayedon the thus prepared roll in accordance with conventional metal sprayingpractice and a deposit of aluminum of uniform thickness of, about .020inch was produced. The coated roll was subjected to a temperature of1600 F. for five hours in an ordinary, indirectly-fired heat treatingfurnace, under an oxidizing atmosphere containing approximately 15% freeoxygen, the balance largely being free nitrogen. At the end of theheating period the article is permitted to cool in the oven tosubstantially room temperature.

Microscopic examination of the coating on the article revealed threewell-defined aluminum alloy layers of varying aluminum contents from theinnermost to the outermost layer. Hardness tests proved the depositedcoat to approximate diamond hardness. Spectrographic analyses of theouter .0004 inch portion revealed the presence of aluminum oxideprimarily as alpha alumina, iron oxide, chromium oxides and nickeloxides, all of which were contained in a matrix of aluminum-rich alloywhich also contained substantial amounts of the AlzCr. Severalunidentified constituents were detected in this coating surface portion,one of these being in substantial percentage and showing in considerablestrength on the spectrum at the nine angstrom position. This constituentremains unidentified to date.

Chemical analysis of the skin portion of the coating of the aluminumalloy portion revealed that 22.08% of that parts was aluminum oxide,12.16% was nickel oxides (as NiO and NiOz), 1.66% was F6203, 1.38% waschromium oxides, and the balance was metallic, 45.80% being aluminum,11.74% being iron. Silica, silicon and manganese also were present insmall amounts.

Example 11 A roll similar in form to that of Example I, but of differentcomposition, being a chromium alloy steel containing about 12% chromium,was similarly prepared by sand blasting and was metal spray-coated withaluminum to a thickness of about .020 inch. The heat treatment was thesame as that stated in Example I. Metallographic and spectroscopicanalyses of the surface portion of the final article revealed that thecoating was substantially like that of Example I. Chemical analysis ofthe coating revealed that the surface or skin contained 35% aluminaexisting primarily in the alpha form, 3.4% of chromium oxide and 1.5%iron oxide. The balance of this surface portion was metallic aluminumamounting to about 50% of the coating, chromium and iron being presentin lesser amounts but alloyed with the aluminum as in the previousexample.

Hardness of this article proved to correspond closely to that of thearticle of Example I.

Example III Still another roll like that of Example I, but of copperrather than alloy steel, was metal spray-coated with heat softenedaluminum after being prepared by sand blasting. The aluminum coat wasestablished uniformly at 0.010 inch and the roll was subjected to atemperature of about 1450 F. for about five hours under an oxidizingatmosphere as described in Example I.

The final article, after being cooled in air to room temperature, wasexamined microscopically, spectroscopically and chemically. The articlehad three welldefined aluminum-copper alloy layers in the aluminum alloyportion, the outermost layer being the richest in aluminum and theinnermost being the richest in copper. The surface portion (to 0.0004inch depth) of the article contained an alumina and copper oxide, theformer being the preponderant oxide constituent of the surface portion,and contained aluminum-copper alloy as a matrix for the oxides. Thetotal oxide content of this portion amounted to about 35%. Againhardness measured as accurately as possible approximated that ofdiamond.

Example IV A roll like that of the foregoing examples but of mild steelwas sand blasted and then provided with an aluminum coating about 0.030inch thickness by conventional metal spray procedure. After heattreatment involving subjecting the coated article to a temperature ofabout 1700 F. for six hours under an oxidizing atmosphere as defined inExample I, the article was examined as in the previous cases. Hardnessproved to approximate the diamond. Three well-defined aluminum alloylayers were present in the coating and the aluminum content increasedfrom the inner to the outer layer. The surface portion (to 0.0004 inchdepth) proved to contain about 30% oxides, the large proportion of whichwas alumina (mostly in the alpha form). Iron oxides were, however,present in this surface portion in appreciable quantity and all theseoxides were contained in and firmly bonded to an aluminum-rich alloymatrix.

Example V An Inconel roll in form like that of Example I was prepared bysand blasting and then metal spray-coated with aluminum in accordancewith conventional practice, the coat being uniform and about 0.035 inchthickness. Heat treatment as described in Example I but lasting forabout seven hours resulted in a final article of approximately diamondhardness in the aluminum coated portion. Again three Well-definedaluminum alloy layers existed in this portion and these layers as in theforegoing examples were of approximately the same thickness. Thealuminum content of these layers increased from the inner one to theouter and the outer layer had an oxide-rich surface portion (0.0004 inchdeep) in which the proportion of oxide to metal (alloy) amounted toabout 40%. The preponderant oxide was alumina (again primarily in thealpha form) but oxides of nickel and chromium were also present inappreciable amounts. These oxides were securely held in a matrix ofaluminumrich alloy which appeared to be primarily of aluminum andnickel.

As may be deduced from the foregoing: examples, the rate of cooling inthis heat treatment is not critical, but I prefer to cool the articlesin the furnace or in air, for reasons of convenience and economy. Thehardness of the surface of the articles thus prepared is not materiallyimproved by quenching, and while quenching may increase the rate ofproduction, it also requires special materials and equipment and moreoperations than my preferred practice. Heat treatment of special type isnot necessary prior to application of the aluminum.

This is a continuation-in-part of my copending application, Serial No.737,709, filed March 27, 1947, now abandoned in favor of the presentcase.

Having thus described the present invention so that those skilled in theart will be able to understand and practice the same, I state that whatI desire to secure by Letters Patent is defined in what is claimed.

What is claimed is:

l. A corrosion resistant metal article comprising a base portion and analuminum alloy portion bonded tenaciously thereto and comprising aplurality of aluminum alloys of different compositions, said alloyportion being between about .005 inch and about .040 inch thick andhaving an oxide-rich surface portion approaching diamond hardness andhaving a ditfinity for metal particles, said surface portion containingalumina as a preponderant oxide constituent and an oxide of metal of thebase in a matrix of aluminum alloy.

2. A corrosion resistant metal article comprising a steel base portionand an aluminum alloy portion bonded tenaciously thereto and comprisinga plurality of aluminum alloys of different compositions, said alloyportion being between about .005 inch and about .040 inch thick andhaving an oxide-rich surface portion approaching diamond hardness andhaving a difiinity for metal particles, said surface portion containingalpha alumina as a preponderant oxide constituent and iron oxide in amatrix of aluminum alloy. ll

3. A corrosion resistant metal article comprising a copper base portionand an aluminum alloy portion bonded tenaciously thereto and comprisinga plurality of aluminum alloys of different compositions, said alloyportion being between about .005 inch and about .040 inch thick andhaving an oxide-rich surface portion approaching diamond hardness andhaving a diffinity for metal particles, said surface portion containingalumina as a preponderant oxide constituent and copper oxide in a matrixof aluminum alloy.

4. A corrosion resistant metal article comprising a bronze base portionand an aluminum alloy portion bonded tenaciously thereto and comprisinga pluraliy of aluminum alloys of different compositions, said alloyportion being between about .005 inch and about .040 inch thick andhaving an oxide-rich surface portion approaching diamond hardness andhaving a diffinity for metal particles, said surface portion containingalumina as a preponderant oxide constituent and an oxide of metal of thebase in a matrix of aluminum alloy.

5. A corrosion resistant metal article comprising a base portion and analuminum alloy portion bonded tenaciously thereto and comprising aplurality of aluminum alloys of different compositions, said baseportion consisting essentially of a metallic material selected from thegroup consisting of pure iron, commercially pure iron, steels, copper,bronzes, nickel and nickel alloys, and said alloy portion being betweenabout .005 inch and about .040 inch thick and having an oxide-richsurface portion approaching diamond hardness and having a diflinity formetal particles, said surface portion containing alumina as apreponderant oxide constituent and an oxide of metal of the base in amatrix of aluminum alloy.

6. A corrosion resistant metal article comprising a chromium-nickelalloy-steel base portion and an aluminum alloy portion bondedtenaciously thereto and comprising a plurality of aluminum alloys ofdifferent compositions, said alloy portion being between about .005 inchand about .040 inch thick and having an oxide-rich surface portionapproaching diamond hardness and having a difiinity for metal particles,said surface portion containing alpha alumina as a preponderant oxideconstituent and oxides of iron, chromium and nickel in a matrix ofaluminum alloy.

7. A corrosion resistant metal article comprising a base portion and analuminum alloy portion bonded tenaciously thereto and comprising threewell-defined aluminum alloys of different compositions and substantiallythe same thickness, the aluminum content of the alloys of said layersincreasing from the inner layer contacting the base portion to the outerlayer, said alloy portion being between about .005 inch and about .040inch thick and having an oxide-rich surface portion approaching diamondhardness and having a diflinity for metal particles, said surfaceportion containing alumina as a preponderant ox-- ide constituent and anoxide of metal of the base in a matrix of aluminum alloy.

8. A corrosion resistant metal article comprising a base portion and analuminum alloy portion bonded tenaciously thereto and comprising aplurality of aluminum alloys of different compositions, and increasingaluminum content from the inner layer contacting the base portion to theouter layer, said alloy portion being between about .005 inch and about.040 inch thick and and having an oxide-rich surface portion approachingdiamond hardness and having a difiinity for metal particles, saidsurface portion having an oxide content between about and about 40% andcontaining alumina as a preponderant oxide constituent and an oxide ofmetal of the base.

9. A corrosion resistant metal article comprising an alloy steel baseportion containing about nickel and 15% chromium, and an aluminum alloyportion bonded tenaciously to said base portion and comprising threewelldefined aluminum alloys, the aluminum content of the al loys of saidlayers increasing from the inner layer contacting the base portion tothe outer layer, said alloy portion being between about .005 inch to.040 inch thick and having an oxide-rich surface portion approachingdiamond hardness and having a diffinity for metal particles, saidsurface portion containing about 22% alpha alumina, about 12% nickeloxides, about 1.5% chromium oxides and about 1.5 iron oxides, thebalance of the surface being essentially metallic and the metal thereofserving as a matrix for the said oxides.

10. A corrosion resistant metal article comprising a chromium-iron baseportion containing about 12% chro mium, and an aluminum alloy portionbonded tenaciously to said base portion and comprising threewell-defined aluminum alloys, the aluminum content of the alloys of saidlayers increasing from the inner layer contacting the base to the outerlayer, said alloy portion being between about .005 inch to .040 inchthick and having an oxide-rich surface portion approaching diamondhardness and having a diffinity for metal particles, said surfaceportion containing about 35% alpha alumina and about 3.5% chromiumoxides and about 1.5% iron oxide, the balance of the surface consistingessentially of metal serving as a matrix for the said oxides.

11. A metal article for use in high temperature metal working operationscomprising an alloy steel body having a surface portion containingaluminum and approaching diamond hardness and having a difiinity formetal particles, produced by metal spraying an object having achromium-nickel-iron alloy surface with aluminum and there by producingon said object an aluminum coating between about 0.005 inch and about0.040 inch thickness, heating the object to about 1400 F. andmaintaining it at that temperature for about five hours, raising thetemperature of said object to about 1600 F. and maintaining it at aboutthat temperature for about five hours, raising the temperature of saidobject to about 2000 F. and maintaining it at that temperaure for aboutfive hours, and cooling the object in air to room temperature.

12. The method of producing a metal article for use in high temperaturemetal working operations having a working surface approaching diamondhardness and having a ditfinity for small metal particles whichcomprises the steps of metal spraying a metal object with an oxidationresistant alloy steel selected from the group consisting of nickelsteels, chromium steels and chromium nickel steels and thereby coatingsaid object, metal spraying the thus coated object with aluminum andthereby coating the object with aluminum and thereby producing on saidobject an aluminum coating between about 0.005 inch and about 0.040 inchthickness, heating the coated object to about 1400" F. and maintainingit at about that temperature for about five hours under an oxididingatmopshere, raising the temperature of the object to about 1600 F. andmaintaining it at that temperature for about five hours, raising thetemperature of said object to about 2000 F. and maintaining it at thattemperature for about five hours, and cooling the object to roomtemperature.

13. The method of producing a metal article for use in high temperaturemetal working operations having a worksurface approaching diamondhardness and having a diiiinity for small metal particles whichcomprises the steps of metal spraying a ferrous metal object with anoxidation resistant alloy steel selected from the group consisting ofnickel steels chromium steels and chromium-nickel steels and therebycoating said object, metal spraying the thus coated object with aluminumand thereby coating the object with aluminum and thereby producing onsaid object an aluminum coating between about 0.005 inch and about 0.040inch thickness, heating the coated object to about 1400 F. maintainingit at that temperature to about five hours under an oxidizingatmosphere, then raising the temperature of said object to about 1600"F. and maintaining it at about that temperature for about five hours,then raising the temperature of said object to about 2000 F. andmaintaining it at that temperature for about five hours, and cooling theobject in airto room temperature.

14. The method of producing a metal article for use in high temperaturemetal working operations having a working surface approaching diamondhardness and having a diifinity for small metal particles whichcomprises the steps of roughening a surface portion of a ferrous metalobject, metal spraying said surface portion with an oxidiation resistantalloy steel selected from the group consisting of nickel steels andchromium steels and nickelchromium steels and thereby coating saidobject, metal spraying the thus coated object with aluminum and therebycoating the object with aluminum and thereby producing on said object analuminum coating between about 0.005 inch and about 0.040 inchthickness, heating the coated object to about 1400 F. and maintaining itat that temperature to about five hours under an oxidizing at mosphere,then raising the temperature of said object to about 1600" F. andmaintaining it at about that temperature for about five hours, thenraising the temperature of said object to about 2000 F. and maintainingit at that temperature for about five hours, and cooling the object inair to room temperature.

15. The method of producing a metal article for use in high temperaturemetal working operations having a working surface approaching diamondhardness and having a ditfinity for small metal particles whichcomprises the steps of heating a ferrous metal object to a temperaturebetween about 200 F. and 500 F., metal spraying a surface portion ofsaid heated object with an oxidation resistant alloy steel selected fromthe group consisting of nickel steels, chromium steels andchromium-nickel steels and thereby coating said object, metal sprayingthe thus coated object with aluminum and thereby coating the object withaluminum and thereby producing on said object an aluminum coatingbetween about 0.005 inch and about 0.040 inch thickness, heating thecoated object to about 1400" F. and maintaining it at that temperatureto about five hours under an oxidizing atmosphere, then raising thetemperature of said object to about 1600" F. and maintaining it at aboutthat temperature for about five hours, then raising the temperature ofsaid object to about 2000 F. and maintaining it at that temperature forabout five hours, and cooling the object in air to room temperature.

16. The method of producing a metal article for use in high temperaturemetal Working operations having a working surface approaching diamondhardness and having a difiinity for small metal particles whichcomprises the steps of metal spraying with aluminum a metal objecthaving a surface portion of an oxidation resistant alloy steel selectedfrom the group consisting of nickel steels, chromium steels and nickelchromium steels and thereby producing on said surface portion analuminum coating between about 0.005 inch and 0.040 inch thick, heatingthe coated object to about 1400" F. and maintaining it at thattemperature to about five hours under an oxidizing atmosphere, thenraising the temperature of said object to about 1600" F. and maintainingit at about that temperature for about five hours, then raising thetemperature of said object to about 2000" F. and maintaining it at thattemperature for about five hours, and cooling the object in air to roomtemperature.

17. The method of producing a metal article for use in high temperaturemetal Working operations having a working surface approaching diamondhardness and having a difiinity for small metal particles whichcomprises the steps of metal spraying a ferrous metal object with achromium-nickel alloy and thereby producing a coat between about 0.005inch and 0.025 inch thick on said object, metal spraying the thus coatedobject with aluminum and thereby producing a coat of aluminum betweenabout 0.005 inch and 0.040 inch thick on said object, heating the coatedobject to about 1400 F. and maintaining it at that temperature to aboutfive hours under an oxidizing atmosphere, then raising the temperatureof said object to 10 about 1600" F. and maintaining it at about thattemperature for about five hours, then raising the temperature of saidobject to about 2000" F. and maintaining it at that temperature forabout five hours, and cooling the object in air to room temperature.

18. The method of making a corrosion. resistant metal article having asurface approaching diamond hardness and having a ditfinity for metalparticles which comprises the steps of metal spraying with aluminum ametal ob ject and thereby producing on said object an aluminum coatingbetween about 0.005 inch and about 0.040 inch thickness, heating thecoated object for about five hours under an oxidizing atmospherecontaining between about 15% and about 30% oxygen to between about 1400F. and about 1800" F. and alloying the deposited aluminum with metal ofsaid object, and cooling the object to room temperature.

19. The method of making a corrosion resistant metal article having asurface approaching diamond hardness and having a ditfinity for metalparticles which comprises the steps of metal spraying With aluminum aferrous metal object and thereby producing on said olbject an alumi numcoating between about 0.005 inch and about 0.040 inch thickness, heatingthe coated object for about five hours under an oxidizing atmospherecontaining between about 15% and about 30% oxygen to between about 1500"F. and about 1800 F., and furnace-cooling the object under saidatmosphere to room temperature.

20. In combination, a furnace conveyor element having a work-supportingferrous or alloy body, said body having a melting point above about1400" R, an aluminum oxide surface covering the work-engaging portion ofsaid body, said surface being in weld relation to the workengagingexterior portion of said body.

21. In combination, a furnace conveyor element having a ferrous or alloybody, a thin aluminum coating covering said body in bonded relationthereto and having a work-engaging surface consisting essentially ofaluminum oxide, said body having a melting point above about 1400" F.

22. In combination, a furnace conveyor element having a ferrous or alloybody and a thin aluminum coating in weld relation thereto over theinterface between the body and said coating, said body having a meltingpoint above about 1400" F., said coating covering the work engagingexterior area of said body and having a depth greator than about .005inch, said coating further having its work-engaging surface comprisingaluminum oxide.

23. In combination, a furnace for heat treating metal work at 1400" F.or above, at least one conveyor element in said furnace to engage workpassing through said furnace, said conveyor element having awork-supporting ferrous or alloy body, said metal body having aworkengaging portion, said work-engaging portion having an aluminumoxide surface, whereby the formation of accretions is inhibited whenwork engages said aluminum oxide surface.

24. In combination, a furnace for heat treating metal work at 1400" F.or above, at least one conveyor element in said furnace to engage workpassing through said furnace, said conveyor element having awork-supporting ferrous or alloy body, a thin aluminum coating in Weldrelation to said body along the interface between said body and saidcoating, said coating having a Work-engaging surface of aluminum oxide,whereby the formation of accretions is inhibited when Work engages saidaluminum oxide surface.

Gilson Mar. 24, 1914 Van Aller Oct. 5, 1915 (Other references onfollowing page) '12 Whitfield et al. Aug. 1, 1939 Merritt Aug. 29, 1939Axline Nov. 3, 1942 Simmons June 3, 1947 Robertson Dec. 6, 1949

20. IN COMBINATION, A FURNACE CONVEYOR ELEMENT HAVING A WORK-SUPPORTINGFERROUS OR ALLOY BODY, SAID BODY HAVING A MELTING POINT ABOVE ABOUT1400* F., AN ALUMINUM OXIDE SURFACE CONVERING THE WORK-ENGAGING PORTIONOF SAID BODY, SAID SURFACE BEING IN WELD RELATION TO THE WORKENGAGINGEXTERIOR PORTION OF SAID BODY.